The present invention relates to a method for controlling the extinction angle for a line-commutated converter in dependence on a minimum reference value for the extinction angle, and to a device for carrying out the method.
An installation for transmission of high-voltage-direct current between two ac voltage networks comprises two converter stations, each one being connected on its ac voltage side to a respective one of the ac voltage net works, and a common dc connection.
Each one of the converter stations comprises a converter, usually at least one converter transformer for connection of the converter to the ac voltage network, and shunt filters for generating reactive power and for filtering harmonics. The converters are normally line-commutated, current-source converters, by which is to be understood that the current commutation between the valves of the converters, which usually are arranged in six-pulse bridges, takes place by means of voltages occurring in the ac voltage network, and that the dc connection, as viewed from the converters, occurs as a stiff current source. A converter valve usually comprises a plurality of mutually series-connected semiconductor elements, capable of being fired, in the form of thyristors.
During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other, hereinafter referred to as the inverter, operates in inverter operation. Control equipment for the respective converter generates a control signal corresponding to a control angle xcex1, at which firing pulses are supplied to the valves of the converter. For the purpose of minimizing the consumption of reactive power by the converters and reducing the stresses on components included in the converter stations, it is advantageous to control the rectifier with the smallest possible control angle xcex1 and to control the inverter with the smallest possible extinction angle xcex3 (margin of commutation) without jeopardizing the controlled operation. The control system of the installation is therefore usually designed such that the inverter is controlled to a maximum dc voltage which is suitable for the operating conditions of the installation, taking into consideration safety margins with respect to commutating errors, voltage variations on the ac voltage network and other deviations from nominal operation which may occur. The rectifier is controlled in current control, the reference value of which is formed in dependence on a current order, which in its turn is formed in dependence on a power order and the actual dc voltage in such a way that the direct current and hence the transferred power remain at a desired value.
Usually, the control equipment for rectifiers and inverters are designed identical, whereby, in the rectifier, a current controller is activated and, in the inverter, control equipment for a control which aims at maintaining the extinction angle at, but not allowing it to fall below, a preselected minimum value is activated.
For a general description of the technique for transsot mission of high-voltage direct current, reference is made to {dot over (A)}ke Ekstrxc3x6m: High Power Electronics HVDC and SVC, The Royal Institute of Technology, Stockholm 1990, in particular chapter 4.
The current controller in the inverter is supplied, in addition to the reference value of the current in the dc connection and its actual value, also with a so-called current margin with such a sign that the control equipment of the inverter strives to reduce the direct current controlled by the rectifier. During stationary inverter operation, the output signal from the current controller of the inverter will thereby assume a maximum value limited by a limiting signal and the value of the control angle xcex1 ordered by the inverter is determined by the limiting signal.
Between the control angle xcex1, the extinction angle xcex3 and the overlap angle xcexc, during which commutation between two valves takes place, the known relationship xcex1+xcexc+xcex3=180xc2x0 prevails. It is thus desirable, for the inverter, to determine the control angle such that the extinction angle (the margin of commutation) remains at a predetermined minimum value.
U.S. Pat. No. 4,563,732 describes control equipment where the value of the control angle xcex1 ordered by the inverter is formed in dependence on the output signal from an OR circuit. The OR circuit is supplied with the output signal from the current controller of the inverter as well as with a control signal formed in dependence on a continuing predicted value of the extinction angle which would be the result if a commutation were to be started at the calculating instant. The valves of the inverter are thus fired in dependence on the signal which is supplied to the OR circuit earliest.
A control system for determining the above-mentioned predicted value of the extinction angle is further described in an article in a journal, {dot over (A)}ke Ekstrxc3x6m and Gxc3x6te Liss; A refined HVDC Control System. IEEE Transactions on Power Apparatus and Systems, Vol. 59, No. 5/6, June 1970, pages 723-732. The system is based on the fact that a certain voltage-time area is required for carrying out the commutation. The criterion for the predicting control system is that, after completed commutation, the remaining voltage-time area is to exceed a certain prescribed minimum value. In the event that a voltage or current disturbance should occur during a commutation in progress, there is then a possibility of terminating this without any commutating error. A predicting member calculates continuously, by a triangular approximation of the curve shape of the voltage, the total voltage-time area which would remain if the semiconductor elements of the valve were to be fired at the running instant. The calculation is carried out by subtracting, from the continuously calculated triangular voltage-time area, a voltage-time area corresponding to the voltage-time area during the time the commutation process is taking place. This latter voltage-time area is directly proportional to the direct current in the dc transmission. The predicting control system gives a firing signal to the conmuutating valve, that is, the valve which is in turn to take over the current, when the remaining voltage-time area becomes equal to a given reference value, formed in dependence on a given minimum reference value xcex3min for the extinction angle.
In practice, the control equipment also comprises means for controlling that the ac voltage and the minimum control angle lie within the prescribed limits before a firing order is generated.
In {dot over (A)}ke Ekstrxc3x6m: High Power Electronics HVDC and SVC, The Royal Institute of Technology, Stockholm 1990, pages 7-14 to 7-16 and FIGS. 7-10, an embodiment of such control equipment is described, where the extinction angel xcex3 occurs explicitly. By means of a predicting member, containing information about the preceding zero crossing of the voltage, there is predicted continuously, in dependence on sensed values of commutating voltage and current, which commutating margin xcex3pred would be obtained if firing were to take place at the moment of prediction. The predicted commutating margin is compared with a reference value xcex3order for the minimum extinction angle and when the predicted value becomes equal to the reference value, a firing order is generated for the commutating valve.
In the same publication, on pages 7-13 and FIGS. 7-9, also an embodiment of such control equipment is shown, based on negative feedback of a sensed value of the extinction angle. A reference value xcex3order for the minimum extinction angle is compared with the sensed value and when this falls below the reference value, a firing order for the commutating valve is generated. To compensate for the delay caused by the feedback of the sensed value of the extinction angle, an addition to the reference value, formed in dependence on sensed commutating voltage and current, is given via a so-called disturbance detector.
A commutating process between two valves in a line-commutated converter is initiated when the thyristors in the commutating valve are fired. The thyristors in the decommutating valve are then supplied with a negative voltage, given by the ac voltage network, which strives to reduce the current by a rate of change which is determined by the impedance of the current circuit. The current passes through zero but continues for a certain period of time to flow through the thyristor in the inverse direction thereof, whereby both current and voltage across the thyristor have a negative polarity. The overlap angle is measured from the moment the commutation starts until the current through the decommutating thyristor becomes zero and the extinction angle from the moment when the current passes through zero until the voltage across the thyristor passes through zero to assume a positive polarity.
A thyristor is not able to take blocking voltage in the forward direction immediately after the current in the conducting direction of the thyristor has ceased, and the time required for it to recover this ability is referred to as the recovery time of the thyristor. In line-commutated converters, the control equipment must therefore be designed such that the minimum value of the extinction angle accommodates the recovery time of the thyristor. In addition to this, the minimum value must also comprise margins for voltage transients, to ensure a certain control margin for the transmission system in the event of an oscillation arising in the dc transmission, and, of course, also for the individual variations of the thyristors included in the valves of the converter. This generally leads to the above-mentioned minimum value of the extinction angle being set at 15-20xc2x0, normally in excess of 17xc2x0. Typically, about half of this extinction-angle value is due to the recovery time of the thyristor.
With increasing minimum value of the extinction angle follows an increasing consumption of reactive power while at the same time the available dc voltage becomes lower than the theoretically available dc voltage. Also the generation of harmonics increases with increasing extinction angle, both on the dc side of the inverter and on the ac voltage side thereof.
During implementation of the above-described pieces of control equipment with predicting control systems for extinction-angle control, the predicting member is supplied with a certain preselected reference value for the extinction angle, typically of the magnitude xcex3min=16 xc2x0.
This leads to certain disadvantages. At low current a relatively, and probably unnecessarily, large margin towards commutating errors, caused, for example, by a voltage transient, is obtained, whereas inversely, at high current, the converter will operate with an extinction angle which approaches the recovery time of the thyristor, whereby the margin towards commutating errors drops.
The object of the invention is to provide an improved method of the kind described in the introductory part of the description, which permits a continuous adaptation of he extinction angle to operating parameters for the semi-conductor elements in the valves of the converter, and a device for carrying out the method.
According to the invention, this is achieved by continuously forming an operating value of recovery time for the semiconductor elements of a decommutating valve, and by forming the minimum reference value of the extinction angle in dependence on this operating value.
According to an advantageous further development of the invention, the operating value of recovery time is formed in dependence on at least one of a continuously formed operating value of the junction temperature of the above-mentioned semiconductor elements, a continuously formed operating value of the rate of change of the blocking voltage of the operating value in the forward direction, and a continuously formed operating value of the rate of change of the commutated current.
With a method and a device according to the invention, a larger control range for the dc voltage of the inverter may be allowed while at the same time the current dependence of the margin with respect to commutating errors is reduced.